First remove the skill from Alexa (either via Alexa app or website)
Optionally delete any custom lists from Alexa which have previously been synchronised
Next go to
This will show all apps you have given access to your Microsoft account. Find "Tasks In The Hand for Alexa" and click on it and select "remove these permissions"
You are now back to a clean state
Return to Alexa and add the Tasks In The Hand skill. It will take you through the account linking process and re-add these permissions but importantly it will also issue a refresh token to Amazon which will allow it to maintain access automatically.